Piezoelectric relays driven by piezoelectric bending elements may employ a snap-action or bistable device to accumulate energy supplied by the piezoelectric bending element. See Ser. No. 200,390, filed Oct. 24, 1980, now U.S. Pat. No. 4,383,195 incorporated herein by reference. The full drive voltage is applied initially. When sufficient energy is stored, actuation occurs, whereupon the snap-action device produces quick, decisive operation.
In some applications full drive voltage is not initially available. The drive voltage is a slowly varying control voltage, such as encountered in automatic street light systems, which must nevertheless produce a quick, positive actuation when the operating voltage is reached. For example, relays used to turn street lights on and off at dusk and dawn must operate consistently at a predetermined voltage level of the slowly varying control voltage from a photosensitive element. The switching should occur at a relatively high level of illumination well above the condition of total darkness. The switching must be abrupt and positive to prevent contact chatter and consequent arcing and deterioration of the contacts.
Attempts to use a snap-action device in combination with a piezoelectric bending element resulted in less than desired response. The contact force becomes zero before the contacts open and a part of the actuating stroke is dissipated in premature motion as the contacts start to close. The mechanical detenting action of the snap-action device or overcenter spring device is not adequate: it introduces an amount of motion which is significant relative to the available contact stroke.